This study aims to investigate the characteristics of effective interaction between a therapist and his/her child client with autism spectrum disorder during a session of occupational therapy. Four sessions were video-recorded and analyzed, including one conducted by a novice therapist and three by expert therapists. Three children with autism spectrum disorder who were able to take part in everyday conversations also participated in this study. In study 1, we coded the therapists’ utterances according to their function in the context through a session. In study 2, we segmented the video-recording of a session based on the goal of child behavior, recorded the therapists’ physical support, and examined not only the temporal relationship between the therapists’ utterances and physical support but also the children’s proactive behavior. The results indicated that the expert therapists appropriately provided attentive utterances and physical support according to the children’s actions. We discuss a new and useful framework for understanding therapist support and children with developmental disabilities.